It is known in the art that the velocity of a sound wave in a material depends on the mechanical properties of the material.
Bone velocity measurement typically uses one of two methods. In a first method, an ultrasound wave is transmitted across a bone, in a direction transverse to its axis, for example across the phalanx or across an ankle. In a second method, an ultrasonic wave is transmitted from a skin surface generally parallel to the bone, to the bone, and its reflections or emissions from the bone, at a distance along the axis of the bone, are detected.
In order to perform in vivo ultrasonic measurements of the mechanical properties of a bone, it is necessary to transmit an ultrasonic wave through the soft tissue surrounding the bone. Unfortunately, the thickness of the soft tissue varies along the length of the bone. Also, the soft tissue velocity is not a constant value for all soft tissues. These variations can affect the accuracy of the ultrasound propagation time measurement through the bone. Typically, the variations in thickness of the soft tissue and its velocity are either ignored or an attempt is made to cancel the effects of the soft tissue.
For example, U.S. Pat. No. 5,143,072 and PCT publication WO 97/13145, the disclosures of which are incorporated herein by reference, describe methods of overcoming the effects of the unknown thickness of the intervening soft tissue, by ensuring that the measurements are taken when the portion of the path which passes through soft tissue is of a same length for different measurements or by determining a soft tissue velocity.
U.S. Pat. No. 4,819,753, the disclosure of which is incorporated herein by reference, describes a method of analyzing the status of a hip implant, by detecting the time of flight of vibrations from a hammer hitting the bone, at a knee and outside a spina iliaca posterior superior of a pelvis. In this method, very low frequency acoustic waves are generated and detected, between about 50 Hz and 2 kHz.
U.S. Pat. No. 4,048,986, the disclosure of which is incorporated herein by reference, describes a method of diagnosing or identifying a person by measuring the effect on a polarization of an ultrasonic wave which travels between an elbow and a wrist or between a knee and an ankle. Audio-frequency waves appear to be suggested.